This invention relates to a water purification system for purifying raw water such as tap water and groundwater to provide drinking water for domestic or business use.
In a water purification system of the type, sterilization of raw water is carried out in various manners. As a recent technical trend, use has widely been made of a hollow fiber membrane module (commercially available) for destroying microorganisms and bacteria and suppressing growth or propagation thereof. Use is also made of a unit for carry out electrolysis of the raw water and/or another unit for generating an appropriate amount of chlorine.
Generally, the raw water such as tap water and groundwater contains various pollutants, namely, residual chlorine such as hypochlorous acid (HClO.sup.-), musty odor, trihalomethane, chlorinated organic compounds, pigments, and so on. In the water purification system as a water treatment system, the raw water is made to pass through a filter (typically, an activated carbon filter) to adsorb and remove the above-mentioned pollutants as adsorbed substances. Thus, the raw water is filtered or purified through the filter into filtered water.
With the lapse of time, the adsorbed substances will be accumulated in the filter and microorganisms and bacteria will be grown in the filter. The accumulation of the adsorbed substances and the growth of the microorganisms and the bacteria may prevent the raw water from smoothly flowing through the filter. This results in deterioration of the performance of the filter. In this connection, a reproducing operation is carried out after the above-mentioned purifying operation is continued for a predetermined time period. In the reproducing operation, the filter is applied with an electric voltage to generate the Joule heat. Such heat generation at the filter serves to desorb the adsorbed substances from the filter and to destroy and desorb the microorganisms and the bacteria grown on the filter. Desorbed substances as well as destroyed and desorbed microorganisms and bacteria are discharged through a drain pipe communicating with the filter.
However, the conventional water purification system described above has only one filter arranged in the system. It is assumed that the filter is designed for removal of chlorinated organic compounds as a main object and has a plurality of water passages each of which is of a diameter corresponding to the chlorinated organic compounds. In this event, standard plate count bacteria of a smaller size are allowed to pass through the filter. This results in growth of the bacteria in a pipe downstream of the filter.
On the other hand, the reproducing operation periodically performed requires a relatively long time in order to completely remove the chlorinated organic compounds adsorbed to the filter. During the reproducing operation, supply of the purified water is interrupted. In view of the usability, an interval of the reproducing operation can not be so shortened. However, if the interval is prolonged, the growth of the bacteria in the filter may possibly be caused to occur as described above.
In order to avoid the growth of the bacteria during the interval of the reproducing operation, it is proposed to incorporate a sterilizing operation in addition to the reproducing operation. The sterilizing operation is performed by applying an electric voltage to the filter in a short period of time.
However, although the effective destruction of the bacteria is enabled, such incorporation of the sterilizing operation has another problem that the destroyed bacteria will be mixed into the purified water during supply of the purified water following the sterilizing operation.